In recent years, a development of such an image forming apparatus that an electromagnetic wave containing a frequency component of at least a part of a frequency area in a range from a millimeter wave band to a terahertz band (from 30 GHz to 30 THz) is detected and an intensity of each of pixels which are two-dimensionally arranged can be imaged has been started. Such an electromagnetic wave is also simply called “terahertz wave” or he like hereinbelow. Such a development has been started because the terahertz wave has the following nature and is considered to be useful in industries. First, the terahertz wave passes through a non-metallic substance in a manner similar to X-rays. Second, a number of absorption spectra which are peculiar to a biological molecule, medical and pharmaceutical products, or the like exist in such a frequency band. Third, the terahertz wave has space resolution necessary to many imaging uses. From the above feature, as application fields of the terahertz wave, a spectral analysis technique in the substance, a safe fluoroscopic imaging apparatus in place of the X-rays, an analysis technique of a biological molecule or medical and pharmaceutical products, and the like are considered.
As for the detecting element constructing the image forming apparatus in such a frequency area, hitherto, there are many detecting elements using a thermal detection. They are called “direct detection” or “video detection” and are a detecting method which is liable to be influenced by a low frequency noise such as a 1/f-noise or the like in the element. As a detecting method of the electromagnetic wave, a method called a synchronous detection different from the direct detection is known. The synchronous detection is a method whereby the electromagnetic wave to be detected is modulated with respect to time by a RF or the like and the modulation signal detected by the detecting element is demodulated by using a reference signal synchronized with the RF, is returned to a detection signal, and is detected. Generally, such a method is difficult to be influenced by the low frequency noise in the electromagnetic wave detecting element. In recent years, a method other than the thermal detection in the frequency area in the range from the millimeter wave band to the terahertz band has also been developed. Therefore, if such a nature of the synchronous detection is used, there is a possibility that a 2-dimensional array detecting element which can perform image forming and image pickup operations with a low noise and at a high sensitivity is realized.
PTL 1 discloses a method of lastly demodulating modulation signals of all pixels in a lump in an image forming sensor apparatus in a light area. Such a method is very convenient because it can use a commercially available image pickup apparatus as it is. Even suitable conditions are settled, an image can be photographed at a high sensitivity as a real-time moving image as if an ordinary television program was seen. PTL 2 likewise discloses an image forming sensor apparatus in a light area, wherein memory cells are arranged in a pixel and the synchronous detection is performed. Such a restriction of a CCD (Charge Coupled Device) that a signal has to be completely read out of the pixel during the photographing can be avoided. PTL 3 discloses a heterodyne detecting apparatus for forming an image in a millimeter wave band, wherein a weak electromagnetic wave generated from an object can be photographed.